1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to methods for processing signals acquired using streamer cables or receiver station lines having both pressure responsive sensors and motion responsive sensors.
2. Background Art
In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth from an acoustic source and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The seismic wavefield is reflected when there is a difference in acoustic impedance between the layer above the interface and the layer below the interface. When using towed streamers in marine seismic exploration, one or more seismic streamers is towed behind an exploration vessel at a water depth typically between about six to about nine meters, but can be towed shallower or deeper. Hydrophones are included in the streamer cable for detecting seismic signals. A hydrophone is a submersible pressure gradient sensor that converts pressure waves into electrical or optical signals that are typically recorded for signal processing, and evaluated to estimate characteristics of the subsurface of the earth.
In a typical geophysical exploration configuration, a plurality of streamer cables is towed behind a vessel. One or more seismic sources are also normally towed behind the vessel. The seismic source, which typically is an airgun array, but may also be a water gun array or other type of source known to those of ordinary skill in the art, transmits seismic energy or waves into the earth and the waves are reflected back by reflectors in the earth and recorded by sensors in the streamers. Paravanes are typically employed to maintain the cables in the desired lateral position while being towed. Alternatively, the seismic cables are maintained at a substantially stationary position in a body of water, either floating at a selected depth or lying on the bottom of the body of water, in which case the source may be towed behind a vessel to generate acoustic energy at varying locations, or the source may also be maintained in a stationary position.
After the reflected wave reaches the streamer cable, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the hydrophones in the streamer cable. The water surface is a good reflector and the reflection coefficient at the water surface is nearly unity in magnitude and is negative in sign for pressure signals. The waves reflected at the surface will thus be phase-shifted 180 degrees relative to the upwardly propagating waves. The downwardly propagating wave recorded by the receivers is commonly referred to as the surface reflection or the “ghost” signal. Because of the surface reflection, the water surface acts like a filter, which creates spectral notches in the recorded signal. Such spectral notches make it difficult to record data with a broad spectrum. Because of the influence of the surface reflection, some frequencies in the recorded signal are amplified and some frequencies are attenuated.
For pressure recording of vertically propagating waves, maximum attenuation will occur at frequencies for which the propagation distance between the detecting hydrophone and the water surface is equal to an integer multiple of one-half wavelength, the first notch being at zero frequency. Maximum amplification will occur at frequencies for which the propagation distance between the detecting hydrophone and the water surface is an odd number integer multiple of one-quarter wavelength. The wavelength of the acoustic wave is equal to the velocity divided by the frequency, and the velocity of an acoustic wave in water is about 1500 meters/second. Accordingly, the location in the frequency spectrum of the resulting spectral notch is readily determinable. For example, for a seismic streamer at a depth of 7 meters, and waves with vertical incidence, maximum attenuation will occur at a frequencies zero and about 107 Hz and maximum amplification will occur at frequencies of about 54 and 161 Hz.
It is known in the art to use sensor cables deployed on the water bottom (“ocean bottom cables”) which have both pressure responsive sensors such as hydrophones and particle motion sensors, such as geophones, accelerometers or velocity meters. The signals generated by the particle motion responsive sensors are sensitive to the direction from which the motion originates. The pressure responsive sensor signals typically are not directionally sensitive. Such features of particle motion sensors and pressure responsive sensors have been used to attenuate the effects of water layer multiple reflections. See, e.g., U.S. Pat. No. 5,163,208 issued to Barr et al.
More recently, marine seismic streamers have been developed that include both particle motion responsive sensors and pressure responsive sensors. See, e.g., U.S. Pat. No. 7,239,577 issued to Tenghamn et al. and assigned to an affiliate of the assignee of the present invention. Using such streamers is intended to provide techniques for attenuating the effects of the surface ghost. It has been determined through testing and use of streamers such as the one disclosed in the foregoing patent that the signals generated by the particle motion responsive sensors may be subject to towing noise. U.S. Pat. No. 7,359,283 issued to Vaage et al. and assigned to an affiliate of the assignee of the present invention describes methods for using streamers having both pressure responsive sensors such as hydrophones and particle motion responsive sensors to deal with such noise. The techniques include simulating part of a particle motion sensor signal at low frequencies from the pressure responsive sensor signal, using the depth of the marine seismic streamer and the sound wave velocity in water. The simulated low frequency part of the motion sensor signal is combined with the remainder of the motion sensor signal to produce a “full bandwidth” motion sensor signal. The full bandwidth motion sensor signal can be used in conjunction with the pressure signal to determine upgoing and downgoing components of the seismic wavefield.
In performing the method described in the '283 patent, a simplifying assumption is made that all the sensors in the streamers are at essentially the same depth in the water. It is frequently the case that sensors on a streamer are not at the same water depth during operation. It is desirable to have a method to combine pressure responsive seismic signals and motion responsive seismic signals that does not depend on all the sensors being at the same water depth.